Cobra 40X CB Radio Squelch Dynamike RF Gain PA User Manual
- June 13, 2024
- Cobra
Table of Contents
ADDENDUM
TO
SERVICE MANUAL
MODEL 40X
OPERATING THEORY OF PLL FREQUENCY SYNTHESIZER
- Fundamental theory of PLL Circuitry
The purpose of PLL (Phase Locked Loop) circuit is to generate multiple number programable frequencies from a signal reference frequency with quartz crystal accuracy.
A basic PLL circuitry consists of reference oscillator, VCO, phase comparator and DC filter (low pass filter).
With the above circuit the VCO (Voltage Controlled
Oscillator) Frequency is effectively locked to the reference oscillator, and
its accuracy is as good as the reference oscillator.
Since the CB radio’s adjacent channel spacing is 1OKHz (or multiple of SKHz), our purpose should be to produce multiple of programable frequencies that are spaced apart by 10KHz.
Therefore the basic PLL circuitry is expanded as follow:
The most important part of VCO circuitry is a
voltage controlled variable capacitor called varices or varactor diode whose
capacitance depends on DC voltage applied to its cathode.
The
varactor diode is responsible for setting VCO frequency, and once set it
regulates the VCO frequency against the reference. The VCO frequencies are
chosen in 16 to 17MHz range as shown on table 1. To obtain transmit signal the
VCO is mixed with 10.24MHz. As an example for channel 1: 10.24 + 16.725 =
26.965MHz
For receiver mode the VCO is used as a first local oscillator channel 1:
26.965 — 16.27 = 10.69S5MHz
The above first IF of 10.695MHz is mixed again with 10.24MHz crystal
oscillator frequency which serves as the second local oscillator.
10.695 — 10.24 = 0.455MHz
As can be seen above the VCO frequency shifts from 16.725 to 16.27MHz when
changed from transmit to receive for the same channel 1.
The shift is accomplished by “read only memory” incorporated inside the PLL
IC-1 between the selector switch and the VCO divider (programable).
When transmit logic signal is applied to the IC-1 through pin 19, the
programable divider will divide incoming VCO frequency by 3345 to produce SKHz
sampling signal.
16725 +3345 = SKHz
For the receiver mode the programable divider will automatically change to
divide the VCO frequency by 3245.
16270 +3254 = SKHz
Note that the reference frequency of SKHz is obtained by dividing the 10.24MHz
by 2048 times. (SKHz reference is used instead of 1OKHz for division
convenience).
See table 1 for transmit/receive mode VCO frequencies.
- Transmitter Circuit
IC-1 (PLL LSI), VCO section of IC-203 (pin 1, 2 and 3) are operational regardless of the receive or transmit mode. When the radio is set to the transmit mode, mixer/amplifier section of IC-3 (pin 4, 6, 7 and 9), Q206, Q301 , Q302 and Q303 are activated. The VCO frequency selected by the channel selector switch is mixed with 10.24MHz to generate desired transmit frequency. The mixing is done by a balanced mixer circuit located inside the IC3.
Equivalent Circuit of IC203
The resulting transmit frequency from pin 9 of
IC203 is filtered by L301 and L302.
Q301 is an amplifier/switch circuit. When VCO frequency is out of “Lock” condition pin 14 of IC202 pulls down bias voltage of Q301 to ground disabling Q301 from passing possible illegal frequencies.
Q302 is a RF power driver circuit and Q303 is the final RF power amplifier.
A modulation audio signal is applied to the collectors of Q302 and Q303 through a audio power transformer T201.
The audio signal (mic input) applified by a single power IC201.
The modulation limiting is accomplished by a automatic livel control circuit which is as follow:
L305 and C350 are series resonator, and L306, L307, C340, C344 and C345 make
up pie-
low pass filter.
C335 is factory selected and limits the RF output power level to within the
FCC limit of 4
watts.
- Receiver Circuit
In the receiver mode of operation, Q206 transistor is turned off. Also bias voltage is applied to Q105 and a proper bias and AGC voltage is established to Q101, Q102, Q103, and QIOS.
Q101 is a 27MHz RF input amplifier and any excessive input signal is limited by dildes D101 and D105. The amplified 27MHz is mixed with VCO frequency selected by channel switch.
For channel 1 VCO is set at 16.27MHz. The resulting first IF is 26.965 — 16.27 = 10.695MHz.
Q102 and Q103 is the first converter, and the 10.695MHz is sharply filtered by L103 and a ceramic filter CF-1. The first IF is again mixed with a second local oscillator of 10.24MHz.
10.695 — 10.24 = 0.455MHz.
Q104 is the second converter and the 455KHz. Second IF is filtered by a razor sharp ceramic filter of CF-2 coupled with L105.
Q105 is a first 45SKHz amplifier, and the Q106 being the last amplifier.
D108 is a detector diode which produces audio signal as well as a negative DC voltage for
AGC action.
The negative voltage also provides forward biasing to the cathode of ANL clipping diode of D110. The biasing voltage has a time Constance determined by R128 and C124.
Therefor any sharp negative going pulse from D108 will back bias A110 and be clipped.
ALIGNMENT EQUIPMENT FOR CB TRANSCEIVER
-
Test Voltage
DC 13.8V + 5%, unless otherwise specified. -
Test Equipment
All Test equipment should be properly calibrated.
1. Audio signal generator, 1O0Hz-20KHz.
2. VIT’VM ImV measurable
3. DC ampere meter, 2A
4. Regulated power supply, DC 0-20V, 2A or higher
5. Frequency counter, 0-40MHz, high input impedance type
6. RF VTVM probe type
7. Oscilloscope, 30MHz, high input impedance
8. RF watt meter, thermo-couple type, 50 ohm, 5W
9. Standard signal generator, 1OOKHz-SOMHz, 50 ohm unvalanced
10. Speaker dummy resistor, 8 ohm SW
11. Circuit tester, DC, 20K ohm/V -
Transmitter Alignment
3-1 Test Setup
Refer to the diagram shown below.
Transmitter Alignment Set Up
Note:
a. When connecting audio cable to the microphone input circuit, always use
a shield cable.
b. When making alignment for RF power output, always use the supplied DC
cable.
PLL Circuit alignment
a. 10.24MHz Oscillator check
Connect a frequency counter to the pin 12 and check to see 10.240000MHz +
100Hz.
When a defective crystal is replaced, and if the frequency is higher than by
100Hz the C304 should be increased. If the frequency is lower, the C304 should
be reduced in capacitance.
With a factory supplied crystal C304 value of 47 pfd should be sufficient but
on some sets minor value selection may be necessary.
b. VCO alignment
1. Set the Radio to channel 40 and in transmit mode. (make certain 50 ohm
dummy load or wattmeter is connected to antenna terminal)
2. Connect a circuit tester between TPI and ground.
3. Adjust L203 to obtain 5.0V DC.
4. Set the Radio to channel !| and in receive mode.
5. Check to see the TPI DC voltage dropping to a level between 0.3 to 1.0
volt DC. As long as the DC level stays between 5.0V DC for Transmit at channel
40 and 0.3 to 1.0V DC
for receive at channel 1, the VCO is set properly.
The magnitude of the TPI voltage swing is determined by C312 at factory.
The optimum value of C312 was foung to be around 60 pfd.
C312 with value larger than 60 pfd will reduced the voltage swing magnitude
and vice versa for smaller value. If the lower value drops only to 1.5V DC,
then the C312 of 60 pfd should be reduced to increase the range.
But this shouldn’t be necessary when factory supplied parts are used for D303
(varactor diode) and L203 (VCO tuning coil).
3-2 RF Amplifier Stage Alignment
1. Reduce power supply voltage to 9.0V.
2. Place the channel selector in channel 19, and connect the oscilloscop to
the antenna connector through a suitable connection pad.
3. Adjust L204, L301, L302 and L303 for maximum amplitude of the scope
display.
4. Increase the power supply voltage to 13.8V, then adjust L306 until the
watt meter indicates 3.8W.
5. Measure the transmit power output at all channels, and make sure the power
output difference between any channels is less than 0.3W.
6. Measure the transmit frequency at all channels, and make sure the
frequency is within +800Hz from the assigned channel center frequencies.
3-3 Transmit Frequency Check
1. Set the radio into transmit mode with no modulation.
2. Connect the frequency counter to the antenna load or to the tab provided
at the wattmeter.
The frequency should be within t800Hz from each channel center frequency as
tabulated in the frequency table attached.
3-4 Modulation Sensitivity Alignment
1. Set the unit to transmit mode of operation.
Feed 1KHz, 30mV signal to the microphone input circuit, and adjust RV201 so
that 100% modulation is obtained.
2. Next, reduce the signal input level to 3mV, and make sure the modulation
is higher than 60%.
- Receiver Circuit Alignment
Test Setup
Refer to the diagram shown below.
RECEIVER ALIGNMENT SETUP
Sensitivity Alignment
- Set the signal generator to provide 27.185MHz, 1KHz 30% modulation. Place the channel selector in channel 19 position.
- Adjust L101, L102, L108 and L103 for maximum audio output across the 8 ohm dummy load resistor.
This alignment should be performed by gradually decreasing the signal output signal to a minimum level required for tuning to avoid inaccurate alignment due to AGC action.
Squelch Circuit Alignment
-
Set the signal generator to provide 5O0dB, 1KHz, 30%mod. antenna input.
-
Rotate the squelch control in full clockwise direction.
-
Temperarily adjust RV101 for maximum audio output, and note the audio output level.
Then adjust RV101 so that the audio output level decreases by 6dB. -
Next, reduce the antenna input signal level to 43-48dB and make sure the audio output decreases to zero.
-
Reduce antenna signal input level to zero, and adjust the SQ control until the noise output decreases to just disappear.
Frequency Chart
CH NO| CHANNEL
FREQ (MHz)| CRYSTAL
OSC| VCO
---|---|---|---
TX| RX
1| 26.965| 10.| 16.725| 16.27
2| 26.975| “| 16.735| 16.28
3| 26.985| “| 16.745| 16.29
4| 27.005| “| 16.765| 16.31
5| 27.015| “| 16.775| 16.32
6| 27.025| “| 16.785| 16.33
7| 27.035| “| 16.795| 16.34
8| 27.055| “| 16.815| 16.36
9| 27.065| “| 16.825| 16.37
10| 27.075| “| 16.835| 16.38
11| 27.085| “| 16.845| 16.39
12| 27.105| “| 16.865| 16.41
13| 27.115| “| 16.875| 16.42
14| 27.125| “| 16.885| 16.43
15| 27.135| “| 16.895| 16.44
16| 27.155| “| 16.915| 16.46
17| 27.165| “| 16.925| 16.47
18| 27.175| “| 16.935| 16.48
19| 27.185| “| 16.945| 16.49
20| 27.205| “| 16.965| 16.51
21| 27.215| “| 16.975| 16.52
22| 27.225| “| 16.985| 16.53
23| 27.255| “| 17.015| 16.56
24| 27.235| “| 16.995| 16.54
25| 27.245| “| 17.005| 16.55
26| 27.265| “| 17.025| 16.57
27| 27.275| “| 17.035| 16.58
28| 27.285| “| 17.045| 16.59
29| 27.295| “| 17.055| 16.6
30| 27.305| “| 17.065| 16.61
31| 27.315| “| 17.075| 16.62
32| 27.325| “| 17.085| 16.63
33| 27.335| “| 17.095| 16.64
34| 27.345| “| 17.105| 16.65
35| 27.355| “| 17.115| 16.66
36| 27.365| “| 17.125| 16.67
37| 27.375| “| 17.135| 16.68
38| 27.385| “| 17.145| 16.69
39| 27.395| “| 17.155| 16.7
40| 27.405| “| 17.165| 16.71
PARTS LIST FOR COBRA MODEL 40X
Reference No.| Manufacturer’s Part Number| DYNASCAN
Part Number
---|---|---
R104,108| 0111019GR| 182-029-9-001
Q201,202,203| 01120020C| 176-128-9-001
Q101,107,301| 01120030C| 176-115.9-001
Q109,206| 01120060T| 177-049-9-001
Q205| 01120070T| 176-115-9-004
Q106| 01120080H| 176-125-9-001
11103100| 176-028-9-003
Q105| 11200801| 176-115-9-002
11103100| 176.028-9-003
Q110,204| 01120100T| 176-132-9-001
Q303| 01130040E| 172-062-9-001
11600400| 172-024-9-002
Q302| 01130070E| 176-120-9-001
11600100| 172-059-9-001
Q102,103| 01170010B| 182-138-9-001
IC201| 21100300| 307-272-9-001
IC202| 21300400| 307-272-9-002
IC203| 21401200| 307-272-9-003
D102,105,108,111,112,206| 30000600| 150-001-9-005
30000700| 150-016-9-001
D302| 30000800| 152-114-9-001
0101,103,104,106,107,109,| |
110,201,202,203,204,301,
304,305,401,402,403,404,
405,406,407,408,409,410, 411, 412| 30005200| 151-035-9-001
D205,306| 30001100| 151-093-9-001
30001200| 151-028-9-007
30001300| 151-108-9-001
30001500| 151-050-9-001
30001400| 151-082-9-001
D303| 30002100| 154-009-9-001
30002000| 154-008-9-001
D307| 30003000| 158-045-9-001
30004100| 238-019-9-001
X201| 42500200| 132-036-9-001
CF2| 52600500| 140-006-9-001
CF1| 52600800| 140-006-9-002
R318| 1410001JS| 011-001-5-100
R231| 1415002JS| 011-002-5-150
VR104| 15103201B| 008-436-9-001
VR103,105| 15503201B| 008-457-9-001
VR102| 15503211A| 008-436-9-002
RV101,201| 1810308KH| 008-407-9-004
C114| 224710770| 025-182-9-001
C332,350| 261010775| 023-065-9-004
C345| 261810775| 023-065-9-003
C340| 262710775| 023-065-9-002
C344| 263310775| 023-081-9-001
C303,338| 264700775| 023-065-9-006
C302| 265600775| 023-065-9-005
C133| 268090775| 023-081-9-002
CH301| 31000819B| 042-041-9-001
T201| 31001928D| 061-057-9-001
RFC305| 33100100H| 047-008-9-001
RFC304| 33100100V| 047-008-9-002
RFC307| 331091000| 041-134-9-007
RFC202,204,205| 331092000| 041-134-9-006
RFC301,302,303| 332201000| 041-134-9-002
RFC201| 332500050| 041-134-9-001
RFC306| 334540050| 046-039-9.001
R F C309| 335042000| 041-134-9-008
RFC308| 336890050| 041-134-9-004
L108| 340003100| 047-008-9-003
L101| 34000510B| 046-039-9-002
L102| 34000610A| 046-039-9-003
L104,204| 34000710A| 046-039-9-004
L103| 34000710B| 047-008-9-004
L105| 340008100| 046-039-9-005
L106| 340009100| 046-039-9-006
L107
L301
L302| 340010100 34001210A
340013100| 046-039-9-007
046-039-9-008
046-039-9-009
L303| 340014100| 046-039-9-010
L304| 340015100| 046-039-9.011
L307| 34001600B| 046-039-9-012
L305| 34001710B| 046-039-9-013
L306| 340018100| 046-039-9-014
L203| 340035100| 046-039-9-015
PCB 40 Connector| 410212060| 302-670-9-001
PCB Rotary| 41320701 A| 302-672-9-001
PCB LED Display| 41420702A| 302-673-9-001
PCB LED Lamp| 414207030| 302-674-9-001
Microphone Assembly| 501020150| 561-002-9-001
Cover Upper| 711120580| 253-112-9-001
Cover Bottom| 712120660| 252-045-9-001
Bracket 4P Mtg.| 722140880| 250-104-9-001
Name Plate| 784142840| 600-110-9-001
Escutcheon,| 800110390| 380-461-9-001
Lens (Red)| 811140300| 753-013-9-001
Knob (Channel)| 821130010| 751-209-9-001
Knob (Control)| 82213007B| 751-234-9-003
Earphone Jack| 861140170| 773-126-9-001
Meter| 865130010| 320-118-9-001
Speaker 3″ 8 ohm 2W| 865130070| 580-136-9-001
Rotary SW. 40 Ch| 866130020| 083-274-9-001
Push Sw.| 866140100| 088-071-9-001
Slide Sw.| 866140200| 084-117-9-001
Power Cord Assembly| 871130010| 420-040-9-001
Schematic| 933141470| 499-240-9-001